Today, water-absorbing resin and hydrophilic fibers such as pulps are widely used as components of sanitary/hygienic materials (hereinafter, they are collectively referred to as sanitary materials, sometime) such as paper diapers, sanitary napkins, so-called incontinent pads and the like, in order to use the water-absorbing resins and the hydrophilic fibers to absorb body fluids.
As a main raw material of the water-absorbing resins, the following materials are used, for example: a cross-linked polymer containing partially-neutralized polyacrylic acid; starch-acrylic graft polymer; saponificated vinyl acetate-acrylic ester copolymer; hydrolyzed acrylonitril copolymer; hydrolyzed acrylamide copolymer; cross-linked hydrolyzed acrylonitril copolymer; cross-linked hydrolyzed acrylamide copolymer; and cross-linked polymer prepared from a cationic monomer; and the like material.
In case where they have a high water absorbency, these generally-used water-absorbing resins tend to be poor in tolerance against urine (anti-urine tolerance), durability, and temporal stability (stability in terms of changes over time), when they become swelled gels. Therefore, the generally-used water-absorbing resins have such problems that the swelled gels become sticky (slimy), or are liquidized as time passes by. In order to solve the problems, various methods to give the swelled gels a better durability or a better temporal stability have been suggested.
For examples, the following methods are suggested: a method in which a water-soluble ethylene-based unsaturated monomer is polymerized by reverse-phase suspension polymerization in the presence of a cross-linking agent and a water-soluble chain transfer agent (Japanese Laid open patent application, Tokukaihei, publication No. 2-255804; Reference 1); a method in which a highly water-absorbing polymer is prepared by polymerizing, in the presence of a hypophosphorus compound, a water-soluble monomer whose main component is an acrylic acid and its alkali (Japanese Laid open patent application, Tokukaihei, publication No. 2-300210; Reference 2); a method in which a water-soluble ethylene unsaturated monomer is polymerized by aqueous polymerization in the presence of a cross-linking agent and a water-soluble chain transfer agent (Japanese laid open patent application, Tokukaihei, publication No. 3-179008; Reference 3); a method of producing an absorbent by polymerizing, in the presence of a cross-linking agent, a water-soluble radical polymerizable monomer having an acid base or its salt, with a thiol compound having a radical polymerizable double bond (Japanese laid-open patent application, Tokukaihei, Publication No. 9-77832; Reference 4); a method in which a water-absorbing resin is surface-treated, the water-absorbing resin prepared by aqueous polymerization of a hydrophilic unsaturated monomer in the presence of an internal cross-linking agent, and phosphorous acid and/or its salt, the hydrophilic unsaturated monomer containing, as its main component, acrylic acid and/or its alkali metal salt (Japanese laid-open patent application, Tokukaihei, Publication No. 9-124710; Reference 5); and the like method.
In these methods, a chain transfer agent is used in the polymerization so as to change an internal structure of a gel, thereby improving durability of the swelled gel, and temporal stability and anti-urine tolerance of the gel. However, objects of these arts are to improve the gel having a high water absorbency, in terms of durability, anti-urine tolerance, and temporal stability, in which the gel is poor. Therefore, the water-absorbing resins (or absorbents) have insufficient property as a gel. Thus, in order to be used in a water-absorbing material for a paper diaper or the like, these water-absorbing resins need to be improved in terms of properties.
Moreover, in these days there is an upward trend in an amount of the water-absorbing resin per sanitary material in a paper diaper, a sanitary napkin, or the like, and in a percentage by weight of the water-absorbing resin in a water-absorbing material that is constituted of the water-absorbing resin, the hydrophilic fiber, and the like. That is, the water-absorbing material in these days contains the water-absorbing resin in a higher ratio by reducing an amount of the hydrophilic fiber (pulp) having a small bulk density, while increasing the water-absorbing resin having a large bulk density. Hereby, thin sanitary materials are attained without reducing their water absorbency. As a result, the water-absorbing resins should have a function of liquid transportation and liquid distribution, in lieu of the hydrophilic fiber such as a pulp or the like. These functions may be simply referred to as liquid permeability.
In case of the water-absorbing resin in a particle shape, the liquid permeability is considered as an ability of transporting, through insides or gaps of particles, a liquid added onto the water-absorbing resin, so as to three-dimensionally distribute the liquid throughout the water-absorbing resin as swelling the water-absorbing resin. In the gel, which is obtained by swelling the water-absorbing resin as such, this process is carried out by the liquid transportation caused by capillary phenomenon by which the liquid is transported through gaps of particles in the gel.
Moreover, when the liquid passes through the swelled water-absorbing resin, actual liquid transportation follows a rule of diffusion. This is such a slow process that is almost useless to carry out the liquid distribution under the condition under which the sanitary goods are used. On the other hand, in a water-absorbing resin in which the capillary transportation is impossible due to instability of its gel, these materials are so arranged that they are planted in matrix of the fibers so as to separate particles from each other, in order to avoid blocking phenomenon in the gel. Diapers of a new generation has such a structure in which a water-absorbing resin layer contains none or a little amount of a fiber material for assisting the liquid transport. Accordingly, the water-absorbing resin used therein should have a sufficiently high stability when the water-absorbing resin is swelled (to be the swelled gel). With such high stability, the swelled gel would have a sufficient amount of capillary gaps and thus the liquid transport would be allowed in the swelled gel.
In general, in order that a water-absorbing resin may have a high gel strength when swelled, the water-absorbing resin should be prepared from a polymer having a higher cross-linking level. However, the higher cross-linking level inevitably results in loss of swelling volume and retention capacity. Moreover, U.S. Pat. No. 6,403,700 discloses a method of improving a swelling pressure of a gel of a surface cross-linked water-absorbing resin that is neutralized after prepared by acidic polymerization that is carried out at a low neutralization ratio (5 mol % to 30 mol %). However, because it is necessary to deal with the highly acidic polymer, this method has problems in terms of safety and process complexity. Therefore, this method is hard to apply in an industrial production. Moreover, this method cannot attain such a liquid permeability of the water-absorbing resin that allows the structure of the diaper to include a layer solely made of the water-absorbing resin.
Moreover, it is well known that the surface treatment gives the water-absorbing resin a high gel strength. A water-absorbing resin having a high gel strength and a high liquid absorbing ability against pressure is obtained by treating the water-absorbing resin with a surface cross-linking agent of various kinds, or a certain polymer, which are reactive with carboxyl groups of polymer molecules on a surface of the water-absorbing resin. The surface treatment prevents the gel blocking phenomenon.
There are proposed a number of methods to improve the liquid permeability and the diffusibility by modifying the surface of the water-absorbing resin by the surface treatment. Examples of the methods include: a method in which a water-absorbing resin cross-linked via a certain metal ion is used (Japanese laid-open applications, publication Nos. 2002-513043, and 2002-513059); a method in which a water-absorbing resin is improved by a polyamine or a polyimine dissolved in an organic solvent (pamphlet of international publication No. 95/22356); a method in which a water-absorbing resin is surface-treated with a surface cross-linking agent containing a polyol and an aqueous cation (pamphlet of international publication No. 00/53644); a method in which a water-absorbing resin is surface-treated with a surface cross-linking agent containing an aqueous cation and an organic cross-linked compound except polyols (pamphlet of international publication No. 00/53664). It is possible to prevent the gel blocking by these well-known methods. However, in case where the surface treatment is carried out according to these methods, it is not possible to sufficiently improve the diaper having a high water-absorbing resin content in terms of the liquid permeability, especially Saline Flow Conductivity for a sodium chloride solution of 0.69% by weight (Hereinafter, referred to as SFC). Moreover, the surface cross-linking improves the gel strength by increasing the cross-linking density only in a vicinity of surfaces of the particles of the water-absorbing resin, but not interiors of the particles. Thus, the gel strength is not principally improved because the surface cross-linking cannot increase the cross-linking density in the insides of the particles.
Especially, it is well known to improve handling easiness, preservability, or water-absorbing property of a water-absorbing resin powder by adding an inorganic compound to the water-absorbing resin powder.
Examples of the methods include: a method of producing a water-absorbing resin by dry-blending the water-absorbing resin produced with a multivalent metal salt such as aluminum sulfate and then causing the water-absorbing resin to contact with a binding agent (such as water), whereby the water-absorbing resin becomes elastic and difficult to cause the gel blocking (pamphlet of international publication No. 98/48857); a method in which a water-absorbing resin is applied with a mechanical stress by an Osterizer blender or the like after mixed with a permeability retaining agent (such as silica, alumina, titania, clay, an emulsified polymer, suspension polymer, or the like) by a Vortex mixer (pamphlet of international publication No. WO 01/66056); a method in which a surface-treated water-absorbing resin having a specific gel strength is coated with structural or electrostatic spacer; a method of producing a highly water-absorbing resin material containing a highly water-absorbing resin and particles that are agglomerations of hydrate oxide containing two types of metals M1 and M2 at least partially having -M1-O-M2 bond (Japanese laid open patent application, publication No. 10-147724).
By these well known methods, it is possible to prevent the gel blocking. However, with these methods the water-absorbing resin cannot have sufficient liquid diffusibility, especially SFC, to be used in a diaper, or can be used in the diaper only if a very large amount of a liquid permeability improver such as organic or inorganic particles is added therein. Therefore, these methods have a dusting problem (flying of powder, clogging of filters and the like problems in productions of the water absorbent and the diaper). Therefore, it is necessary to improve the water absorbing resin (or water absorbent) in terms of safety and cost.